<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            display: flex;
        }

        div {
            display: flex;
            height: 400px;
            width: 500px;
            /* 侧轴方向垂直居中 */
            /* align-items: center; */
            /* 从上往下排 */
            /* align-items: flex-start; */
            /* 从下往上排 */
            /* align-items: flex-end; */
            /* 拉伸 沿侧轴方向拉伸 默认的 */
            align-items: stretch;
            align-items: center;
            background-color: pink;
        }

        div span {
            /* height: 50px; */
            width: 100px;
            background-color: purple;
        }
    </style>
</head>

<body>
    <div><span>1</span><span>2</span><span>3</span><span>4</span></div>
</body>

</html>